Electrophotographic images are typically produced by first uniformly charging a primary imaging member such as a photoconducting web or drum using known means such as a corona or roller charger. An electrostatic latent image is then formed by image-wise exposing the primary imaging member using known means such as optical exposure, laser scanners, or LED arrays. The electrostatic latent image is then rendered into a visible image by bringing the electrostatic latent image into close proximity to marking particles, alternatively referred to as toner particles, which have been electrically charged so that they will be attracted to the regions of the primary imaging member bearing the electrostatic latent image. Charging the marking particles, which may or may not comprise a colorant such as a dye or a pigment, and bringing the particles into close proximity with the primary imaging member, is generally accomplished using a magnetic brush development station. The marking particles are first rendered suitable for use in a magnetic brush development station by mixing the marking particles with so-called carrier particles. The carrier particles comprise suitable material that will be attracted to the magnets in the magnetic brush development station and may comprise known materials such as ferrites or iron oxides, etc. The carrier particles often comprise various charge agents that impart a controlled charge on the marking particles. The marking particles may also comprise suitable charge control agents so that, upon mixing with the carrier particles, the marking particles obtain an electrical charge of suitable magnitude and sign so as to make them attractive in the proper amounts to the electrostatic latent image in suitable quantities to enable various image densities to be developed in the electrostatic latent image.
In magnetic brush development, toner particles are generally mixed in the sump of the magnetic brush development station with carrier particles to a predetermined level that is measured with a toner concentration monitor. The marking particles are charged by contacting the carrier particles and brought into close proximity to the primary imaging member bearing the electrostatic latent image by rotating the cylindrical shell, the coaxial magnetic core, or both of the magnetic brush development station. The brush is electrically biased in such a manner that, depending on the sign of the charge of the toner particles, the marking particles can be deposited onto the primary imaging member in either the electrically charged or the electrically discharged regions to render the electrostatic latent image visible.
The toned image is next transferred to a receiver, which could be either a final receiver material such as paper, transparency, etc. or to an intermediate transfer member, such as a compliant intermediate transfer member, and then from the intermediate transfer member to the final receiver member. Transfer can be accomplished by applying pressure between the receiver and either the primary imaging member or the intermediate transfer member. More commonly, pressure is applied in conjunction with either an applied electrostatic field or with heat that softens the toner particles. The image is then typically permanently fixed to the final receiver member using pressure, heat, or solvent vapors. Most commonly, the image is fixed to the final receiver by pressing the image-bearing final receiver member against a heated fuser roller. To prevent the final receiver member from adhering to the heated fuser roller, the heated fuser roller is conventionally first coated with a release agent such as a silicone oil. Alternatively, release agents, and in particular wax particles, may be incorporated into toner particles to facilitate release of a fused toner image from the heated fuser roller.
In such systems, it is important that marking particles be electrically insulating when used in conjunction with magnetic brush development and electrostatic transfer. If the particles are not electrically insulating, their charges can change when in contact with the receiver or in the development station. This could impair transfer and development as the applied electrostatic force used to urge the marking particles towards the primary imaging member or to or from a receiver member would vary with the charge on the marking particles. Moreover, even if the charge did not reverse sign or become so significantly altered so as to prevent development or transfer, the control of either or both of these operations could be impeded, resulting in incorrect amounts of marking particles being deposited, with corresponding undesirable density variations and other artifacts occurring.
Printing processes serve not only to reproduce and transmit objective information, but also to convey esthetic impressions, for example when coffee-table books are printed or else in pictorial advertising. An immense problem here is posed in particular by the reproduction of metallic hues. Metallic hues are only imperfectly reproducible by a color mixture formed from primary colors, especially the colors cyan, magenta, yellow, and black (CMYK). A gold tone is particularly difficult to reproduce by means of such a color mixture. It has therefore been proposed to incorporate metallic pigments or particles in printing ink in order that a metallic color may be brought about directly. This in practice has been used in many commercial liquid printing inks. But in the case of electrophotographic toners, where magnetic and/or electrical and especially electrostatic properties are decisive, this is particularly problematic, since metallic constituents may have an adverse effect on these properties.
Nevertheless there have already been proposals in the art to imbue toners with metallic constituents. For instance, U.S. Pat. No. 5,180,650 discloses providing a toner composition, which contains lightly colored metallic constituents, such as copper, silver, or gold for example, in a coating, which in turn has been provided with an over-coating comprised of a metal halide. But the appearance of prints in particular may be adversely affected by chemical reactions of the metallic constituents due to the halides, which can promote oxidations of the constituents for example. For instance, the tarnishing with which everyone is familiar from copper or silver objects may occur, making the metallic quality unattractive or disappear completely. Moreover, these toners are only lightly metallically colored, which is insufficient to reproduce a gold tone in printed matter.
Further, when metallic constituents are incorporated in toners using conventional manufacturing processes, these metallic flakes are typically randomly oriented throughout the toner particles. This random orientation leads to the loss of metallic hue, and causes a rather dark appearance when such toners are fixed to a receiver sheet using heated fusing rollers.
More recently, there have been proposals to modify the surface of metallic flakes such that it becomes hydrophobic and non-conductive, in order to be used in electrophotography. U.S. Pat. No. 7,326,507, incorporated herein by reference for all that it contains, discloses the preparation of a toner for producing a metallic hue. Metallic pigment particles are coated with a silicate followed by an organic layer, and the resulting particles are combined with toner materials. However, the toner was not shown to contain encapsulated metallic flakes in the polymeric resin. Thus, there is a possibility that the metallic pigment itself may be detached from the polymer during the particle making process, resulting in inhomogeneities in the toner that can cause transfer and cleaning problems.